The number of features you have turned on, and average packet size are a starting point for consideration here.
The 560 with 4.0 can maintain 90+Mbps (line-rate) down to an average packet size of about 800 bytes with default settings.
ie. a default NAT firewall. As average packet sizes go below that, throughput drops off fairly much linearly (constant packet rate),
until you get to 8.5Mbps @ 64 bytes.
So if your average packet size is around 180 bytes, then 24Mbps is exactly what you'd expect the 560 to peak out at.
Conventional wisdom is that web traffic and e-mail in a 'standard mix' product 250-350 byte averages. With VoIP and other
protocols driving this down in recent years in a heavily site-dependent manner.
One of the reasons we've added packet-count info to the System Status in 4.5 is to be enable you carry out these kinds of
calculations more easily.
Extra features turned on will have an impact on performance too, so if 180 bytes average packet size doesn't sound right,
that's the next thing to think about.
Thats because you are not looking at version 4.5 beta
Ah! fair point, I guess its worth me upgrading just so i can take a look at it?
There's always the 4.5 emulator which you can play with (see link towards the top of the main summary page) so you can see what's what before upgrading your 565.